Unified atomic mass unit
The unit used to describe the mass of an atom is the unified atomic mass unit, symbolized as amu or μ.
The standard used for measuring the mass of an atom is a 12C atom. Carbon-12 (12C) is an isotope of Carbon with 6 protons, 6 neutrons, and 6 electrons.
The unified atomic mass unit is defined as exactly 1/12 th the mass of one Carbon-12 atom.